Damage to the part of the brain that controls social emotions changes the way people respond to thorny moral problems, demonstrating the role of empathy and other feelings in life-or-death decisions.

Asked to resolve hypothetical dilemmas -- such as tossing a person from a bridge into the path of a trolley to save five others -- people with damage to their ventromedial prefrontal cortex tended to sacrifice one life to save many, according to a study published Wednesday by the journal Nature.

People with intact brains were far less likely to kill or harm someone when confronted with the same scenarios.

The study suggests that an aversion to hurting others is hard-wired into the brain.

The findings could not be used to predict behavior, Damasio said, because the scenarios presented in the study were unrealistic. More research is needed to determine if people with and without brain damage would react differently when faced with real-world moral dilemmas, he said.

A finding linking a specific type of brain damage to day-to-day moral behavior could have legal implications in criminal cases. But researchers said the study was meant only to explore the psychological underpinnings of moral actions, not to characterize decisions as right or wrong.

The ventromedial prefrontal cortex processes feelings of empathy, shame, compassion and guilt. Damage to this part of the brain, which occupies a small region in the forehead, causes a diminished capacity for social emotions but leaves logical reasoning intact.

Researchers from USC, the University of Iowa, Harvard University and the California Institute of Technology posed 50 hypothetical scenarios to six people whose ventromedial prefrontal cortices were damaged by strokes or tumors. Their responses were compared with those given by 12 people without brain damage and 12 others with damage in brain areas that regulate other emotions.

Researchers found no difference among groups in their responses to scenarios with no moral content, such as deciding to turn a tractor left to harvest more turnips.

Scenarios that did not require participants to directly kill or harm someone elicited similar responses among the groups. For example, people, regardless of whether they had brain damage, said they would classify personal expenses as business expenses in order to lower their taxes.

Additionally, members of all groups rejected decisions that would harm someone for the personal benefit of another, such as killing a newborn a parent couldn't care for.

But people with damage to their ventromedial prefrontal cortex were about three times as likely to sacrifice one person for the greater good compared with people without brain damage or those with damage in a different part of their brains.

Joshua Greene, a Harvard psychologist not involved in the research, said the study showed that moral judgment is shaped by two brain systems -- one focused on intuitive emotional responses and another that controls cognition.

"When one of those systems is compromised, decisions are skewed," he said.